Discusses, in some depth, technical examination, cleaning and restoration of this picture, which proved unexpectedly complex. An x-ray mosaic (reproduced in a full page plate) indicated that some of the arrows piercing S. Sebastian's body had been painted out. During cleaning it became clear that an earlier restorer had made extensive alterations to S. Sebastian. Indeed, a completely new left arm and hand had been painted over the original. The whole configuration of the upper arm and body had been changed, some of the original paint scraped away and some limbs made thicker. Details are given of the careful removal of such additions and figures are included in the text which show details of cleaning tests in key areas. Also deals with the technical examination of the pigments, materials used and the paint structure. An interesting comparison of technique and materials is made between this and Sassetta's work.